The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . ucture has developed, historically,from that simplest form of primitive intestine whichwas possessed by our gastrsead ancestors, and which theextant gastrula now exhibits. We have already shown (inChapter VIII.) that the peculiar Hood-gastrula (Amphi-gastrula) of Mammals (Fig. 277) may be referred backto the original BeU-gastrula (Archigastrula) form, which,among Vertebrates, is now accurately retained solely bythe Amphioxus (Fig. 276; Plate X. Fig. 10). Like the latter, the gastrula of Man and
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . ucture has developed, historically,from that simplest form of primitive intestine whichwas possessed by our gastrsead ancestors, and which theextant gastrula now exhibits. We have already shown (inChapter VIII.) that the peculiar Hood-gastrula (Amphi-gastrula) of Mammals (Fig. 277) may be referred backto the original BeU-gastrula (Archigastrula) form, which,among Vertebrates, is now accurately retained solely bythe Amphioxus (Fig. 276; Plate X. Fig. 10). Like the latter, the gastrula of Man and of all Mam*mals must be regarded as the ontogenetic reproductionof that phylogenetic evolution-form which we call theGastraea, and in which the whole body of the animal isintestine. The peculiar form and mode in which the complex DEVELOPMENT OF THE INTESTINAL CANAL. 321 human intestinal canal develops from the simple gastrulaand which is similar to that in other Mammals, can there-fore be only correctly understood when it is considered inthe light of Phylogeny. We must, accordingly, distinguish. Fig. 276.—Archigastrnla of Amphioxus (in longitudinal secticn): d,primitive intestine ; 0, primitive mouth ; i, intestinal layer; e, skin-layer. Fig. 277.—Amphigastrula of Mammal (in longitudinal section). Theprimitive intestine (d) and primitive mouth (0) are filled up by the cells ofthe intestinal layer (i) ; e, skin-layer. between the original primary intestine (the primitiveintestine, or protogaster) of the Skull-less Animals{Acrania), and the differentiated or secondary intestine( after intestine, or metagaster ) of the Skulled Animals(Craniota). The intestine of the Amphioxus (the repre-sentative of the Acrania) forms no yelk-sac, and develops,palingenetically, from the entire primitive intestine of thegastrula. The intestine of the^ Skulled Animals, on theother hand, has a modified, kenogenetic form of evolution,and differentiates at a very early period into two differentpar
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